We are studying the expression of mRNAs that are abundant in the mouse liver. Our objective is to analyze transcription rates of these sequences during development and to determine the frequency of the genes coding for these mRNAs as it has been shown previously that some elements in the abundant mRNAs are repeated in the genome. During the past year we have determined the time course of appearance of MUP mRNA during development. This time course parallels the time of onset of puberty. We have now obtained a bank of DNAs cloned from mouse liver mRNA. We will determine which of these correspond to specific abundant liver mRNAs, try to identify which proteins they specify, and use the labelled cloned DNAs to determine the repetition frequency of the genes they represent and the time course of appearance of their corresponding mRNAs during development. Also we have cloned middle repetitive genes from genomic liver DNA. A number of these genes are expressed in liver and kidney mRNA populations. Initially we will determine the repetition frequency of the genes represented by these clones and try to identify the proteins they specify.